An organic electroluminescent device in the simplest structure is generally constituted of a light-emitting layer and a pair of counter electrodes sandwiching said light-emitting layer and functions by utilizing the following phenomenon. Upon application of an electrical field between the electrodes, electrons injected from the cathode and holes injected from the anode recombine in the light-emitting layer and the energy level after the recombination returns from the conduction band to the valence band with release of energy in the form of light.
In recent years, organic thin films have been used in the development of EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been focused on commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer of an aromatic diamine and a light-emitting layer of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer has been investigated, but these compounds merely produced luminance at an extremely low level. Thereafter, europium complexes were tried to utilize the excited triplet state, but they too failed to emit light at high efficiency. As is cited in the patent document 1, a large number of proposals have been made on the phosphorescent dopants.                Patent document 1: JP2003-515897 A        Patent document 2: JP2001-313178 A        Patent document 3: JP2002-305083 A        Patent document 4: JP2002-352957 A        Patent document 5: JPH11-162650 A        Patent document 6: JPH11-176578 A        Patent document 7: JP2003-142264 A        
What is proposed for a host material to be used in the light-emitting layer in the development of organic EL devices is CBP or a carbazole compound presented in the patent document 2. Since CBP is characterized by having a good hole transfer property but a poor electron transfer property, the use of CBP as a host material for tris(2-phenylpyridine)iridium (hereinafter referred to as Ir(ppy)3), a green phosphorescent emitter, disturbs balanced injection of electrical charges and causes excess holes to flow out to the side of the electron-transporting layer. As a result, the luminous efficiency from Ir(ppy)3 decreases.
One of the means to solve the aforementioned problems is to provide a hole-blocking layer between the light-emitting layer and the electron-transporting layer as described, for example, in the patent document 3. This hole-blocking layer accumulates holes efficiently in the light-emitting layer and contributes to improve the probability of recombination of holes and electrons in the light-emitting layer and enhance the luminous efficiency. Currently, the hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato-N1,O8) aluminum (hereinafter referred to as BAlq). These materials are able to prevent electrons and holes from recombining in the electron-transporting layer. However, BCP tends to crystallize even at room temperature and lacks reliability as a hole-blocking material and the life of the device is extremely short. BAlq is reported to have a Tg of approximately 100° C. and provide a relatively long life, but it has an insufficient hole-blocking ability and the luminous efficiency from Ir(ppy)3 decreases.
On the other hand, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as TAZ) presented in the patent document 4 is also proposed for a host material of phosphorescent organic EL devices; however, since TAZ has a good electron transfer property but a poor hole transfer property, the light-emitting region is shifted to the side of the hole-transporting layer. Therefore, it is conceivable that some of the materials used for the hole-transporting layer have a problem in compatibility with Ir(ppy)3 and decrease the luminous efficiency from Ir(ppy)3. For example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as NPB), which is used most widely in the hole-transporting layer from the viewpoint of high performance, high reliability, and long life, shows poor compatibility with Ir(ppy)3; hence, the problem with NPB is that the energy transfer occurs from Ir(ppy)3 to NPB and the luminous efficiency decreases.
Furthermore, the aforementioned BAlq that has an adequate electron transfer property is proposed for a host material of phosphorescent organic EL devices in the patent document 7. The document states that a phosphorescent organic EL device of long life can be realized without complicating the layered structure; however, it cannot be said that the use of BAlq in the proposed manner is sufficient for practical use.
Moreover, the patent documents 5 and 6 disclose some indolocarbazole compounds, but not the compounds of this invention. These documents recommend the use of the indolocarbazole compounds disclosed therein as a hole-transporting material, but contain no account to teach their use as a phosphorescent host material.